The most critical part of an electric lamp or light bulb is its filament. It is almost always made of tungsten because tungsten has the highest melting point of all known practical materials. Electric lamps are ordinarily gas-filled in order to reduce the rate of vaporization of the tungsten filament, and in gas-filled lamps, efficacy in producing light is increased by coiling the filament. In the common sizes of domestic incandescent lamps, the so-called bread-and-butter lamps, a coiled coil design is generally used.
In low cost mass-produced incandescent lamps, the filament is attached to the lead wires in the simplest manner possible. The ends of the filament are placed in open hooks formed on the ends of the lead wires, and these hooks are then folded back and closed by automated apparatus. The folded hooks pinch the filament and since the coiled end are deformed as a result, the process must be restricted to ductile filaments which have not been heat-treated to achieve recrystallization.
In higher wattage or higher quality lamps such as halogen cycle and photo-projection lamps, greater filament stability together with longer life duration are sought. In such lamps filaments are used which have been recrystallized by firing prior to mounting in order to eliminate internal strains and expel impurities. By so doing, the shape and dimensions of the filament are stabilized for subsequent lamp operation. However such filaments are more brittle and the brittleness makes the attachment of the filament to the inleads or to the lead wires of the lamp frame structure much more difficult. Since the simple hook-clamp is ruled out, various alternative attachment techniques have been developed and that most commonly used is known as spudding. It involves forcing a wire or spud into the end turns or coils of the filament to achieve a tight friction fit. In a spudded joint the electrical connection is essentially a mechanical contact.
Various improvements to the simple spud have been made over the years. U.S. Pat No. 2,449,679--van Horn (1948) uses a spud in which screw threads have been formed either by a screw cutting operation or by winding fine wire tightly around it. U.S. Pat. No. 2,830,217--Hodge (1958) uses a spud in which the end of the conductor is flattened to provide a spade-like tip in order to effect a force fit with the filament coil. Attempts have also been made to combine spudding with welding, for instance in U.S. Pat. No. 2,403,070--Fulton (1946). In the Fulton patent, in order to assure that the portion of the filament that takes the stresses and strains be free of embrittlement due to welding, the coiled filament is welded to the spud at the end remote from the entry point only.
All spudding techniques and improvements thereon, including spudding combined with welding, are essentially labor-intensive and have not been amenable to automation.